Based on current projections from global climate models (GCM's), regional climates in the coterminous U.S. are expected to become warmer and either wetter or drier over the next century depending on the GCM used to make projections. Forest communities and the species that comprise them are likely to respond to a changing climate in a number of different ways based on environmental tolerances that have evolved over the past several thousand years. If, as many scientists believe, global warming is occurring at a rate that is unique in the recent history of the Earth, many species and plant communities are likely to be stressed by temperature and moisture conditions unlike those in which they have evolved. Concurrently, some species and communities in boreal and cold temperate biomes may benefit from warmer temperatures and greater CO2 availability resulting in more successful reproduction, higher growth rates and increased competitiveness. Plant species and communities are likely to respond differently to climate change on different landscapes and at different scales, and therefore a multi-scale, ecoregional approach will be required to understand potential impacts of climate change on species, communities and entire ecosystems. This study is part of a broader effort by the U.S. Department of Defense to assess the vulnerability of military lands to rapid climate change and develop mitigation strategies to cope with projected impacts to natural systems, resource management activities and military missions. The Holdridge Life Zone system was used to model the geographic extent of present and future climatic envelopes that influence the distribution of forest biomes and tree species in the coterminous U.S. The Holdridge system integrates mean annual temperature, mean annual precipitation and mean annual potential evapotranspiration to define bioclimatic life zones that are strongly correlated with the spatial distribution of major forest cover types and tree species distributions. Climate projections were based on an ensemble of 16 GCM's and three future greenhouse gas emissions scenarios (low-B1, moderate-A1B and high-A2). Changes in the extent and location of Holdridge life zones over approximately 80 years were analyzed and results interpreted in terms of potential impacts to forest tree species and major forest cover types. The magnitude of change from historic conditions also was evaluated for 663 U.S. military installations to aid in the development of vulnerability metrics for Department of Defense facilities and to better understand potential climate trajectories for different regions of the country. Cluster analysis was used to group installations on a regional basis and regional variation in projected climate conditions and assessed relative to important resource management issues at representative installations. Forest cover was modeled at Ft. Drum, New York to illustrate potential changes in species composition and cover type distribution at a landscape scale under future climate change scenarios. Stand ages were estimated using data on site index trees available in the Forest Inventory and Analysis (FIA) database for New York. Ecological types were developed from large scale soil survey data (Natural Resource Conservation Service, Soil Survey Geographic Database, SSURGO) and stand-level forest inventory data available from the natural resources program at Ft. Drum. Stand age, ecological type, species life histories and soil properties were used to parameterize a stochastic forest landscape simulation model using the LANDIS-II application and project changes over 80 years under three future CO2 emissions scenarios. Results showed that there is potential for significant changes in the distribution of some tree species and forest cover types at Ft. Drum under the warmer climate conditions projected for the northeastern U.S. Cover types characterized by species at the northern end of their ranges (e.g., species associated with oak (Quercus rubra, Q. alba)-hickory (Carya cordiformis) forest) increased in abundance, especially on more xeric sites such as sand plains and convex landforms covered in coarse-textured glacial till. However, boreal and cool temperate species, such as sugar maple (Acer saccharum), yellow birch (Betula alleghaniensis), aspens (Populus tremuloides, P. grandidentata) and eastern hemlock (Tsuga canadensis) that are major current components of the northern hardwood-hemlock cover type therein, were projected to remain significant components of the Ft. Drum landscape late into the century on all but the most xeric sites. Overall, changes in species composition were less dramatic than expected at a landscape scale and highly sensitive to establishment probabilities related to specific site characteristics (e.g., soil texture and drainage). The lack of a strong climate response at Ft. Drum may be due to the presence of a number of widely distributed tree species with presumed large climatic tolerances and the relatively homogeneous biophysical conditions that exist within this landscape. / Doctor of Philosophy / The Holdridge Life Zone system was used to model the geographic extent of present and future climates that influence the distribution of forest biomes and tree species in the coterminous U.S. The Holdridge system integrates mean annual temperature, mean annual precipitation and mean annual potential evapotranspiration to define bioclimatic life zones that are strongly correlated with the spatial distribution of major forest cover types and tree species distributions. Climate projections were based on an ensemble of 16 GCM’s and three future greenhouse gas emissions scenarios (low-B1, moderate-A1B and high-A2). Changes in the extent and location of Holdridge life zones over approximately 80 years were analyzed and results interpreted in terms of potential impacts to forest tree species and major forest cover types. The magnitude of change from historic conditions also was evaluated for 663 U.S. military installations to aid in the development of vulnerability metrics for Department of Defense facilities and to better understand potential climate trajectories for different regions of the country. Forest cover was modeled at Ft. Drum, New York to illustrate potential changes in species composition and cover type distribution at a landscape scale under future climate change scenarios. Results suggest that there is potential for significant changes in the distribution of some tree species and forest cover types at Ft. Drum over the next 50 to 100 years under warmer climate conditions projected for the northeastern U.S. Warm temperate tree species at the northern end of their ranges (e.g., oaks, hickories) increased in abundance, especially on more xeric sites such as sand plains and convex landforms covered in coarse-textured glacial till. However, boreal and cool temperate species, such as sugar maple, yellow birch, aspens and eastern hemlock were projected to remain significant components of the Ft. Drum landscape late into the century on all but the most xeric sites. Overall, changes in species composition were less dramatic than expected at a landscape scale and highly sensitive to establishment probabilities related to specific site characteristics (e.g., soil texture and drainage).